In recent years, there has existed a continuous trend toward building smaller, faster electronic devices (e.g., transistors). Today extremely small and extremely fast devices are created on integrated circuits that allow for powerful yet small and lightweight computing components. Typically these electronic devices are transistors, for example metal-oxide-semiconductor (MOS) transistors and similar transistors that may include other insulators as well as oxide.
One issue that arises from building ever smaller and faster devices is that these devices often tend to allow some leakage of current through the device when the device is biased OFF. For example, in an MOS transistor, leakage of current can occur through the channel that resides between the source and drain due to the very short length of the channel. Similar issues arise in other types of devices. Construction of small devices often requires a trade off in that very fast devices tend to have high leakage current, while devices with lower leakage current tend to be somewhat slower (e.g., very fast MOS transistors often have lower threshold voltages, and therefore more residual mobile charge in the channel at a zero gate voltage, which produces higher leakage current, while slower devices often have higher threshold voltages and thus less leakage current). Additionally, very fast devices tend to use higher amounts of power than slower devices (e.g., slower MOS transistors have higher threshold voltages and therefore less power is drawn when they are biased OFF and the current and power are lower when the device is biased ON).
The parameters of devices such as MOS transistors are typically configured by controlling the threshold voltage of the channel between the source and drain terminals. One factor typically used to determine the threshold voltage is the doping levels chosen during the manufacturing process. The doping process can typically require multiple masks and multiple dopant additions to achieve the desired device characteristics, which can increase the cost to manufacture the device. Additionally, as devices continually become smaller, statistical variations between devices typically become increasingly larger. Often it is difficult to control parameters such as the doping concentrations and channel length on very small devices with sufficient accuracy to ensure that the desired device characteristics are achieved. Furthermore, over time the device characteristics can change. Time as well as thermal and electrical exposure can alter the threshold voltage of a device.